1. Field of the Invention
The present invention relates to a polishing compound used for the mirror finishing of the plate type work piece such as silicon wafer to which very precise flatness and parallelism are required. Further, the present invention relates to a method of polishing a silicon wafer using said polishing compound.
2. Description of the Prior Art
Recently, the required capacity for IC (integrated circuit), LSI (large scale integrated circuit) or VLSI (very large scale integrated circuit) is becoming bigger and bigger along with the tendency of remarkable progress of an electronic device. Therefore, the more precise surface flatness of semiconductor substrate are becoming necessary. Further, from the view point to improve the productivity, the development of a polishing compound which can perform higher removal rate and a polishing method by said polishing compound are desired. Furthermore, recently, since the products using epitaxial wafer are becoming the main product, width of circuit pattern to be written on the surface of wafer is becoming narrower. The contamination by metal on the surface of wafer, especially, by specific heavy metal such as copper, iron and nickel can be a ground to cause a serious defect at the later epitaxial process, therefore it is very important to reduce the contamination by heavy metal of silicon wafer and semiconductor device substrate at the processing and at the transportation.
In general, the polishing process to obtain a mirror finished surface on a flat plate type material such as glass, metal, semiconductor, ceramics, carbon material or silicon wafer which is the starting material of IC and LSI is carried out by following procedure. That is, a double sided or single sided polishing machine having an upper and a lower platen or one platen whose surface are covered by a thin layer of non-woven cloth is commonly used, and at the actual use, the work-piece is pressed against the platen which rotates by constant speed, providing the aqueous dispersion of polishing compound containing fine particles of colloidal silica. The processing method, conditions and apparatus to improve machining accuracy and removal rate and to obtain a mirror finished surface which does not have metal contamination and fine defects are continuously investigated. Further, to obtain more precise machining accuracy, the purity and stability of polishing compound must be controlled more seriously and other additional conditions must be also more seriously controlled.
As mentioned above, as a polishing compound for silicon wafer, the alkaline aqueous solution based polishing compound in which very fine colloidal silicon oxide fine particles (colloidal silica) are dispersed is generally used. Regarding the size of particle, the adequate size is selected and the precise classified particle which is ordinarily distributed is used. In a case of the silicon wafer polishing, the average center size of colloidal silica is from several nm to several hundred nm, and they are dispersed by stabilized condition in an alkaline aqueous solution. And, said dispersion is a an aqueous slurry in which fine particles of abrasive are dispersed by from several % to several ten %, and a dispersing agent, alkaline agent, organic solvents and other additives are added at need. The contained abrasive fine particles has an ordinary distribution curve having a center at the nominated size, and can contain coagulated particles.
The reason why the polishing compound is based on an alkaline aqueous solution, is that the polishing of silicon wafer is proceeded utilizing the chemical action of the alkaline to the silicon wafer. The thin corroded layer is formed by the corrosive action of alkaline and is removed by the mechanical rubbing action of fine particles of colloidal silica, which is so called mechano-chemical action. Therefore, since said action is largely effected by pH of the polishing compound, it is desirable to use the polishing compound in which buffering solution to stabilize pH is added. And as the buffering solution, a combination of strong base e.g. a hydroxide of alkali metal such as KOH or NaOH or an organic amine and acid, or a salt composed by weak acid and strong base are used.
Up to the present, many kinds of compound are proposed as a polishing compound of a silicon wafer. For instance, silica sol and silica gel are proposed as a polishing compound in U.S. Pat. No. 3,170,273. Further, the technique that the removing speed can be improved by adjusting pH of the colloidal solution within the range from 10.5 to 12.5 is disclosed in U.S. Pat. No. 3,328,141. In U.S. Pat. No. 4,169,337, the technique to add amines in a polishing compound is disclosed. In Japanese Patent Laid Open Publication 2-158684, a polishing compound composed by water, colloidal silica, water soluble polymer of bigger than 100,000 molecular weight and water soluble salts is disclosed. Further, in Japanese Patent Laid Open Publication 5-154760, a method for polishing by the use of a polishing compound containing 10.about.80 wt. % of piperazine which is a kind of water soluble amines to silica of silica gel or silica sol is disclosed. In Japanese Patent Laid Open Publication 9-193004, an aqueous polishing compound in which fumed silica of average primary diameter of particles is 5-30 nm and average diameter of coagulated particles is 30-100 nm whose pH is desirably adjusted within the range of 8-13 is disclosed. The main object of these disclosed methods is to obtain an excellent surface roughness and to improve the removal speed. However, from the view point not to cause a problem of heavy metal contamination, these mentioned methods are not sufficient. Therefore, these mentioned methods can not be used as a polishing compound for the processing of an epitaxial wafer or a semiconductor device which dislikes the heavy metal contamination.
As the origin which causes above mentioned heavy metal contamination, a polishing pad to be used, a piping, a working environment, an apparatus and tools for transportation, a working wear and an operator can be mentioned. However, since this operation is carried out in a high class clean room, and an apparatus and tools are the special contamination controlled type for clean room use, the main origin which causes said heavy metal contamination problem is the polishing compound. In the meanwhile, the removal of metal impurity contained in the starting material of the hydroxide of alkali metal and the salt of it with weak acid and refining is very difficult. Regarding the organic amine, since it forms easily a complex with heavy metal element, it is very difficult to be refined. That is, it is very difficult to remove heavy metal contamination from these compounds and to obtain highly refined products. The permitted quantity level of heavy metal contamination in an epitaxial wafer or in a semiconductor device is ppb (parts per billion) level. Meanwhile, since in the conventional type polishing compound, heavy metal is contained by ppm (parts per million) level as the contamination, it is possible to cause a heavy metal contamination problem on the surface of polished wafer.
The inventors of the present invention, have carried out an intensive study to overcome the above mentioned problems of conventional polishing compound for silicon wafer has, and have found that by use of the buffering solution composed by a specific type of quaternary ammonium and a specific type of weak acid a polishing compound slurry of low heavy metal contamination, with stabilized pH and higher removal rate can be obtained, and accomplished the present invention. That is, the object of this invention is to provide a polishing compound of low heavy metal contamination by which an excellent surface roughness and higher removal rate can be obtained. And another object of this invention is to provide a polishing method by use of said polishing compound.